Method for making a foam seismic streamer

ABSTRACT

A neutrally buoyant seismic hydrophone streamer is constructed by extruding a syntactic foam material comprising an elastomeric material and gas filled microspheres onto a central stress member to form an elongated streamer member. The streamer may then be covered with a suitable water and oil resistant, abrasion resistant covering and be provided with exteriorly affixed hydrophones, or hydrophones can be affixed to the foam core and an outer protective sheath can be extruded or otherwise provided around the streamer and the hydrophones to provide a uniform diameter streamer assembly. Extrusion is effected utilizing a suitable elastoplastic material which can be extruded at pressures below about 300 psi to avoid bursting the gas filled spheres of the syntactic foam.

United States Patent [1 1 Davis Aug. 19, 1975 [54] METHOD FOR MAKING AFQAM SEISMIC 3,531,760 9/1970 Whitfill, Jr. 340/7 R STREAMER 3.699.23710/1972 Melia 174/1015 75 Inventor: Billy w. Davis, Flagstaff, Ariz.OTHER PUBLICATIONS [73] Assign: schlumberger TechnologyE1n1l2(J5l'fl:;l.1OUma1 of Cellular Plastics. January 1965,

Corporation, New York, NY.

[22] Filed: Oct. 26, 1972 Primary Examiner-Ronald W. Griffin Attorney,Agent, or Firm-David L. Moseley; William Appl' 301p," R. Sherman;Stewart F. Moore Related U.S. Application Data [62] Division of Ser. No.105.547, Jan. 11, 1971, Pat. No. [57] ABSTRACT 3744916 A neutrallybuoyant seismic hydrophone streamer is constructed by extruding asyntactic foam material [52] U.S. Cl. 264/453; 161/DIG. 5; 174/1015;comprising an elastomeric material and gas filled mi- 264/DIG' 6;349/711; 3 3 264/46? crospheres onto a central stress member to form an[51] Int. Cl. 601v l/O H04b 13/02 elongated streamer member The streamermay the 0' Search 41, 6; be covered with a i l water and resistant161/016 5; 340/7 R; 174/1015 abrasion resistant covering and be providedwith exteriorly affixed hydrophones, or hydrophones can be af- [561 cledfixed to the foam core and an outer protective sheath UNITED STATESPATENTS can be extruded or otherwise provided around the 2,806,2549/1957 Craig v 264/47 streamer and the hydrophones to provide a uniform2,978,340 4/1961 Veatch et a1. 264/D1G. 6 diameter streamer assembly.Extrusion is effected util 2 Ill/1962 rt t a1- u 264/41 lizing asuitable elastoplastic material which can be ex- 3,272,9l0 9/1966 Flint174/1015 tmded at pressures be|ow about 300 i to avoid 3372370 3/1968340/7 R bursting the gas filled spheres of the syntactic foam. 3,434.1043/1969 Stapleton et a1. 340/7 R 3,477.96? 11/1969 Resnick 260/25 B 5Claims, 3 Drawing Figures 2 Q i 13 5a 55 4a 1 PATENTEB AUG] 91975 FIG IMETHOD FOR MAKING A FOAM SEISMIC STREAMER This application is a divisionof application Ser. No. 105,547, filed Jan. ll, l97l, now US. Pat. No.3,744,016, issued July 3, 1973.

BACKGROUND OF THE INVENTION The present invention relates generally tomarine seismic exploration and more particularly to the provision ofseismic hydrophone streamers that are neutrally buoyant in water tofacilitate towing thereof at any desired water depth withoutnecessitating the provision of externally connected buoyant devices.

In the seismic mapping of substrata under bodies of water such as theocean, one successful method has been the towing of elongated hydrophonestreamers that may reach l(),() feet in length at various depths behinda vessel. Hydrophone transducers are disposed at intervals along thelength of the streamer and are adapted to detect waves reflected fromsubstrata beneath the surface of the body of water. The seismic wavesmay be sonar waves generated by the towing vessel or may be generated byexplosions or other artificially produced acoustical disturbances.

When a hydrophone streamer is not neutrally buoyant in water it must beeither provided with external weights or floats to enable it to remainsubstantially suspended at any suitable water depth. Such streamerconstructions are not generally acceptable because the equipmentnecessary for neutral buoyancy creates water noises that interfere withthe hydrophone response and create false signals.

Some types of hydrophone streamers are limited as to depth of usagebecause the static head of deep water severely reduces the sensitivityof the hydrophones. This adverse condition generally develops due toover compression of the streamer which retards transmission of sonarwaves to the hydrophone transducer and prevents free response of thetransducers thereby reducing the effective signal emitted therefrom.

While a number of different types of streamers have been manufacturedcommercially, the most prevalent type of hydrophone streamer ismanufactured by providing an elongated flexible tubular conduit withinwhich is disposed a plurality of spaced hydrophone transducers and asignal wire bundle establishing electrical connection between thevarious transducers and signal receiving circuitry disposed aboard thetowing vessel. To produce neutral buoyancy the tubular con duit isgenerally filled with a liquid of low specific gravity, such as oil orkerosene. in order to define an overall streamer constructionapproaching the specific gravity of the water in which the streamer istowed, to provide a substantially neutral buoyancy enabling the streamerto be towed without necessitating the provision of ex ternal weights orbuoyant devices to achieve the state of neutral buoyancy. Such ahydrophone streamer of substantially neutral buoyancy may be towedthrough the water at any desired depth in a substantially noisefree.efficient, and smooth manner as compared with streamer structure havingexternal floats. weights, or the like connected thereto.

While neutrally buoyant oil or kerosene filled hydrophone streamerconstructions generally solve streamer noise problems and eliminate thenecessity for expensive weights or buoys, such streamers are notgenerally satisfactory from the standpoint of repair or operation. Inorder to maintain maximum seismic sensitivity. the flexible conduit musthave a relatively thin wall in order to allow efficient transmission ofseismic waves therethrough. Thin walled hydrophone streamers are subjectto rupture thereby allowing contamination of the oil or kerosene with acorrosive fluid such as sea water which obviously results incontamination of and damage to the sensitive electrical hydrophonesdisposed within the conduit structure. Damage to the streamer componentsby the corrosive effects of sea water encroachment may render thestreamer construction beyond repair.

Another disadvantage in the use of oil or kerosene filled streamers isthe depth limitations that must be placed thereon because of theinstability or loss of seismic efficiency that is created when fluidfilled streamers are subjected to severe hydrostatic pressures.

It is also obvious that the signal bundle and the hydrophone structuresthemselves must be protected from damage by the oil or kerosene disposedwithin the tubular conduit. The cost of manufacturing a seismichydrophone streamer of this nature is obviously adversely effected bythe protective measures necessary to eliminate or reduce the potentiallydamaging effects of both sea water externally of the streamer and oil orkerosene disposed within the streamer structure.

Hence, substantial research has been undertaken in order to attempt thedevelopment of generally neutrally buoyant streamer constructions thatare sufficiently flexible to allow spooling thereof for storage inaddition to being substantially incompressible in order to allow towingthereof at any suitable water depth. One approach to solving the problemis set forth in US. Pat. No. 3,480,907 and involves providing agenerally flexible tubular conduit structure enclosing a signal wirebundle, and hydrophone transducers. A mixture of a liquid silicone baseelastomer is mixed with glass or plastic air filled particles in theform of microcapsules or microspheres and pumped into a streamer sectionin much the same way that oil is pumped into oil filled streamersections. The combination of such a plastic with discrete gas filledmicrospheres produces a foamlike mixture referred to as syntactic foam.After cure, the elastomer becomes a substantially incompressible solidbut retains its elasticity and flexibility providing a streamer that issubstantially neutrally buoyant in sea water. Since the elastomer itselfis insufficient to provide sufficient longitudinal strength in thestreamer, a plurality of elongated stress members extend through thetubular conduit into which the elastomer was pumped in order to enhancethe tensile strength of the streamer construction. But these stressmembers were inclined to be excessively compressed or stressed when thestreamer was wound about a spool for storage purpose. The stress memberswould frequently break through the outer skin or more likely into theelastomeric material causing a violent motion frequently resulting inthe breakage of signal wires and/or puncturing of the flexible conduitencapsulating the streamer structure. In addition. by virtue of theexpense of the silicone elastomer, these silicone rubber based syntacticfoam streamers have a high manufacturing cost thereby rendering themundesirable from an economic standpoint particularly for large diameterstreamers.

SUMMARY OF THE INVENTION With the view toward overcoming theaforementioned difficulties in the manufacture, use. and commercialacceptability of seismic hydrophone streamers, the present invention hasfor its primary object the provision of a novel seismic hydrophonestreamer construction utilizing syntactic foam material extruded uponand around a central tensile stress bearing member, to minimize internalstresses encountered upon windup of the streamer. A plurality ofhydrophone transducer assemblies can then be affixed to the streamer inany one of a number of ways to provide a hydrophone streamer ofsubstantially neutral buoyancy when disposed in water.

It is another object of the present invention to provide a novel seismichydrophone streamer construction utilizing a particulate then'noplasticmaterial that may be mixed with gas filled microcapsules or microspheresand extruded at extremely low extruder pressures in order to preventcrushing of the microcapsules during the extrusion process.

It is another important object of the present invention to provide anovel seismic hydrophone streamer construction having a central stressmember that is suitably flexible and may be wound about a conventionalspool for storage purposes without subjecting the streamer constructionto excessive mechanical stresses that might otherwise damage thestructure of the streamer.

Among the several objects of the present invention is noted thecontemplation ofa novel seismic hydrophone streamer construction that issubstantially incompressible thereby allowing the streamer to be towedat any desirable water depth without excessive compression that couldreduce the seismic efficiency thereof.

It is also an object of the present invention to provide a novel methodof manufacturing a seismic hydrophone streamer wherein discrete gasfilled microparticles and particulate thermoplastic material are admixedin the heating zone of an extruder and are extruded at low extrusionpressures below about 300 psi to prevent crushing of the microparticlesduring the extrusion process.

It is among the several objects of this invention to provide a novelseismic hydrophone streamer that is simple in nature, reliable in use,and low in cost.

The above and other objects are achieved by the novel streamer of theinstant invention which comprises a syntactic foam extrudate having acentral stress member, the extrudate being formed by extruding asyntactic foam mixture comprising preferably a thermoplastic elastomericmaterial mixed with small gas filled microcapsules or microspheresdirectly onto a central stress member, such as a cable or the like. Theextrusion process is conducted at relatively low pressures less thanabout 300 psi so as to avoid rupture of the microspheres. A suitableplastic material capable of extrusion at such low pressures, usuallybelow about 300 psi. should be used. For example. a thermoplasticelastoplastic polymer formed from methylene-bis-4- phenyldiisocyanateand polytetramethylene glycol can be used.

The resulting neutral buoyancy streamer can then be provided with asuitable exterior sheath of abrasion resistant, water and oil imperviousmaterial and hydrophones may be affixed to the exterior thereof. In sucha streamer assembly. the signal wires would be disposed exteriorly ofthe streamer itself and held closely to the streamer with clips or thelike. Such streamers are often employed in static arrays or in likeapplications where the streamer is not towed and hence turbulence andnoise resulting from turbulence is not a problem. Alternatively,suitable transducers or hydrophones could be disposed as desireddirectly on the foam core and a suitable sheath material provided aroundboth the foam core and the hydrophones to produce a uniform diameterstructure suitable for towing. In a third, preferred embodiment, acentral stress member surrounded by an appropriate signal wire bundle isprovided and the hydrophone transducers are mounted directly thereon andare interconnected with the signal wires. This assembly is then passedthrough an extruder and the syntactic foam is extruded around thecentral member, the signal wires, and the hydrophones to define anextrudate containing all the essential elements of a seismic streamer.Regardless, however, of the disposition of the signal wires and theouter sheath of the streamer, the streamers of this invention have acentral tensile stress member.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will be betterunderstood by reference to the drawings which are illustrative ofspecific embodiments. In the drawings, like reference numerals areemployed to designate like parts.

FIG. I is an elevational view schematically illustrating the manufactureof a seismic hydrophone streamer by the extrusion process in accordancewith the present invention.

FIG. 2 is a fragmentary sectional view of a seismic hydrophone streamerconstructed in accordance with the present invention and showingemployment of alternative structures.

FIG. 3 is a sectional view of the streamer of FIG. 2 along line 3-3representing a modified embodiment of the present invention.

DETAILED DESCRIPTION OF THE SPECIFIC AND PREFERRED EMBODIMENTS Referenceis made to FIG. 1 which discloses, in schematic form, an extrusionprocess for the manufacture of seismic hydrophone streamers inaccordance with the present invention. An elongated cable or likecentral stress member of sufficient tensile strength to support aseismic hydrophone streamer that may be on the order of 10,000 feet inlength is fed from reel I0 and is spirally wrapped with a plurality ofwires 13 which will constitute the signal wire bundle of the streamer.This wrapping of signal wires is accomplished by methods known in theart. I-lydrophone transducers 38 are then affixed to member I2 and areinterconnected with the signal wires.

The streamer is manufactured by passing the central stress member I2with the wires I3 and hydrophones 38 in place through an appropriateaperture formed in the extrusion die lb of an extruder l8 capable ofextruding the syntactic form material. The aperture is provided withgates (not shown) to permit passage of the hydrophone into the diewithout permitting plastic to escape rearwardly, though the hydrophonesare only slightly larger in diameter than member 12 with signal wires13.

Suitable apparatus must be provided to obtain the admixture of plasticmaterial and gas filled microcapsules or microspheres in the desiredproportions for the foam before the extrusion is undertaken. Forexample, a feeder system to the extruder may include a hopper 20containing the microspheres or microcapsules which are metered into feedhopper 24 by means of a suitable feed mechanism 22. In addition, theplastic material, preferably in the form of flakes or chips ofthermoplastic polymer, is held in hopper 2] and fed to feed hopper 24 bycontrol mechanism 23. The illustrations in FIG. 1 are schematic, and itwill be appreciated that physical admixture of substances prior toextrusion is wellknown in the art and can be accomplished in a number ofways.

Once the materials are fed in the desired proportions to feed hopper 24,they may be mixed by a suitable mixer such as shown by blade mixer 25 tointersperse the microspheres uniformly through the plastic material.

From feed hopper 24, the mixture of the microspheres and plasticmaterial is fed to one end of extruder 18. A screw extruder isillustrated, although it will be appreciated that any extruder mechanismin which the pressure may be controlled to avoid crushing themicrospheres may be used. Extruder screw 26 is rotated by anappropriated motor 28, and the decreasing pitch of the extruder bladesas well as the increase in diameter of the central member 27 of theextruder screw further admix and pressurize the plasticmicrospheremixture as it proceeds through the extruder. The material is melted inheating zone 30 as it enters the extruder and proceeds through theextruder in a progressively more molten state. The blade action on themolten mixture of plastic and microcapsules further serves to homogenizethe mixture.

The relative proportions of microspheres and plastic will of course bedetermined based upon the relative density of each of these ingredientsand the desired amount of buoyancy desired in the ultimate streamer. Inthis regard it is usually desirable to provide a mixture which willproduce a syntactic foam having positive buoyancy since it will benecessary to overcome the negative buoyancy of the central stressmember, the hydrophone assemblies which may be affixed to the streamerand the like. Generally a syntactic foam having a specific gravity ofabout 0.75 to about 0.85 is desirable.

The gas filled microspheres which may be used in accordance with theinstant invention are known in the art. These microspheres may range insize from about 25 to about 300 microns in diameter, and usually have awall thickness of several microns. The microspheres generally have aspecific gravity ofless than'about 0.40, though some have specificgravities of about 0.20 or less. Inasmuch as the plastic materialsuseful in accor dance with this invention will generally have a specificgravity slightly greater than l.0, typically up to about 1.2 or L25, itwill be appreciated that the relative amounts of the plastic and of themicrospheres will vary depending upon the specific gravity of each andthe specific gravity ultimately desired in the syntactic foam extrudateproduct. It should be generally noted, however, that as the relativeamount of the microspheres in the syntactic foam increases, reducing theoverall specific gravity of the foam, the compressive strength andcompressive modulus of the syntactic foam product will decrease.

The plastic ingredient of the syntactic foam used in this invention is aresinous material capable of extrusion, and preferably a thermoplasticresin. Of course the suitable materials must demonstrate flexibility inorder that the ultimate streamer construction has the necessary flexiblecharacteristics. However the plastics, once formed, will provide anessentially noncompressi ble though flexible product. The plastics mustalso be capable of extrusion at comparatively low pressures, less thanabout 300 psi, since at higher pressures microspheres in many cases willbe crushed during the extrusion operation thus destroying the syntacticnature of the foam and any buoyant effect which could be ob tained fromit.

It will be understood that in the extrusion operation the thermoplasticmaterial being extruded with the microspheres is in a molten state andis comparatively fluid. Indeed under such conditions, at extrusionpressures, the microspheres could be crushed despite the fact that afterthe extrusion operation is completed and the plastic resolidifies, thesyntactic foam will be capable of sustaining higher pressures withoutlosing buoyancy from crushing of microspheres. The comparativeincompressibility of the plastic material gives the syntactic foam thischaracteristic and after extrusion is complete this solid, comparativelyincompressible plastic surrounds the microspheres and assists insupporting them against pressure. Thus is provided a comparativelyincompressible material which is lighter than water and remains capableof withstanding high pressures such as will be encountered in underseawork without loss of buoyant characteristics.

Accordingly, it will be appreciated that the extrusion must be conductedat pressures lower than normal pressures used in plastic extrusionprocesses which can be 2,000 pounds per square inch or more. A suitablethermoplastic compound must be therefore selected which is extrudable atrelatively low pressures, below the crushing point of the microspheres,generally below 300 psi. Suitable thermoplastic polymers include forexample linear polyurethanes formed from a difunctional isocyanate and apolyether glycol. The use of such glycol compounds in polyurethanesappears to give these polymers properties enabling their extrusion incomparative low pressures. The resulting polymers are strong yetflexible.

For example, a thermoplastic linear polymer of polytetramethylene etherglycol and methylene bis-4- phenyldiisocyanate, which has been referredto as a thermoplastic elastoplastic' can be used. This urethane polymermay be extruded at pressures less than about l00 psi. Such compositionsare currently sold under the name Roylar A-863 and Roylar A-863 FR bythe Uniroyal Corporation.

Other compositions may also be used provided they may be extruded atpressures which can be withstood by the microspheres. Plasticizers maybe employed to plasticize the thermoplastic resins to enable theirextrusion at lower pressures though such plasticizers can reduce thestrength of the polymer with which they are used.

It is also necessary to heat the thermoplasticmicrosphere mixture duringthe extrusion operation. The urethane polymers as outlined above can beextruded at maximum nozzle temperatures of between about 350F and 400Fwhich can be sustained by the microspheres. Extreme temperatures notsustainable by the microspheres must be avoided. In this regard it willbe appreciated that microspheres composed of glass. ceramic or anappropriate thermoset resin which would be unaffected by thesetemperatures must be used. For example. microspheres of polyethylene ora like thermoplastic which would soften at the extruder temperaturesmust be obviously avoided. Glass microspheres are preferred.

Prior to extruding the syntactic foam mixture onto central stress member12 it may be desirable to prepare the surface of central stress member12 to form a good bond with the plastic material. It is desirable toclean all grease and dirt from the exterior of member 12 and to etch orotherwise provide a roughened surface on member 12 if such does notalready exist by virtue of its cable structure. It also may be desirableto coat the stress member and the signal wires (which are alreadycovered by their own insulative covering) with an adhesive to assist thebond to the syntactic foam. For example, if a urethane plastic is used,the cable may be coated with a urethane adhesive as known in the art.

The melted and homogenized mixture of thermoplastic and microspheres isthen extruded through aperture 34 into die cavity 36 through whichstress member 12 passes with the signal wires and hydrophones affixed.As the central member 12 is fed through die cavity 36, the syntacticfoam mass is formed and cools around member 12, in the interstices ofwires 13, and around the hydrophones 38 (shown in dotted outlinedownstream from extruder l8) producing a syntactic foam extrudate 40wherein a syntactic foam core surrounds central stress member 12. Manyplastics must cure following extrusion to reach their maximum strengthand henceit may be desirable to permit the extruded product to cure fora time at ambient conditions prior to further treatment.

ln alternate embodiments. the hydrophones may be affixed to a likesyntactic foam extrudate with a central stress member in any one ofseveral ways.

l-lydrophone transducers which may be used with seismic streamers or instatic arrays or the like include transducers which operate uponproperties of piezoelectricity. magnetostriction or electrostriction.Piezoelectric crystal transducers are illustrated in the drawings thoughit will be understood that any suitable transducer assembly can be used.

The piezoelectric transducers often comprise. for example. piezoelectriccrystal cylinders which may be slipped over the signal wire bundle andmounted at appropriate intervals as desired. Alternatively thetransducer may comprise a split crystal, which generates electricalsignals in response to the pressure fluctuations resulting from soundwaves or other seismic waves impacting against the crystal duringoperation and can be assembled onto signal wire bundle or onto theextrudate as pointed out below at the desired intervals. The electricalsignals generated by the crystals are transmitted to appropriateelectrical circuitry, typically aboard the towing vessel, by the signalwires 13.

In the constructional method illustrated in FIG. 1 a plurality ofseismic hydrophone transducers 38 can be attached to the exteriorperiphery of the signal wires and connect with the signal wires withleads 65 to produce an extrudate 40 which encapsulate these elements.Subsequently both the foam extrudate containing the hydrophonetransducers may be passed through the die 48 of a second extruderillustrated generally at 50. Additional thermoplastic material suitablefor extruding a protective coating which is both abrasion resistant andwater and oil impenneable is disposed within hopper 52 of the extruderS0 and is fed by gravity to screw 54 driven by motor 56. The material isheated and forced through opening 58 to die 48 where it surrounds theextrudate 40 to produce a uniform thickness covering 64. The outercovering material of the streamer can be any one of many of a plasticmaterials exhibiting abrasion resistance and water and oilimpermeability. Such materials are well-known in the art. it may bedesirable to provide a fibrous net or mesh around extrudate 40 as itpasses through extruder 50 to provide a fibrous reinfrocement for thecovering material 64 or to use fibrous material with the coveringmaterial. It may be desirable to provide cable member 12 with a covering77 to prevent damage to the signal wires by abrasion.

The extrusion of a coating or outer sheath on the exterior of cable orthe like is well-known in the art. Polyethylene, polyamides.polyvinylchloride, synthetic rubber or the like are often used. Much thesame process can be used to extrude outer sheath 64 on the foam as usedto extrude the extrudate 40.

in a typical example, the central stress member 12 could be a cableabout A inch in diameter. The syntactic foam is deposited as a inchlayer around member 12 and signal wires 13. The outer sheath has athickness of about /8 inch, thus producing a streamer having an overalldiameter of on the order of l inch. Of course. the invention is equallyadaptable to streamer constructions of varying size.

Alternatively outer sheath 64 can simply be wrapped and appropriatelysealed onto the extrudate 40 and the transducers to provide a generallyuniform, and typically cylindrical streamer structure. In such a casethe signal wires would extend to the termination of the streamer.

ln a further alternative the syntactic foam can be extruded directly onmember 12 to provide a neutral buoyancy streamer having a central stressmember 12 which can then be provided with protective coating or sheathsuch as 64 and can be used as a streamer onto which are affixed externalhydrophones for use in static arrays or the like.

The syntactic foam extrudate can otherwise be modified to providerecesses 68 therein to accommodate transducers.

In this embodiment the syntactic foam would be extruded directly onmember 12 surrounded by signal wires 13. Recesses 68 would bemechanically cut into the extrudate after its cure and the hydrophones70 be assembled in the recess so that a uniform thickness outer sheath64 could be extruded onto the assembly of transducers on the extrudate.Interconnection of the transducers with the signal wires is accomplishedby cutting through the foam layer and mechanically interconnecting thetransducers to the signal wires 13 with leads 75. Split transducerswhich might be useful in such a streamer are disclosed in applicationSer. No. 101.873 filed Dec. 28, 1970, now US. Pat. No. 3.675.193, issuedJuly 4. 1972. entitled "Hoop Stressed Beam Hydrophoneand filed in thename of Billy W. Davis.

In view of the foregoing it is apparent that there is provided a novelseismic hydrophone streamer construction and syntactic foam extrusionprocess therefor that provide a relatively simple streamer constructionof substantial structural integrity. The streamer construction of thisinvention may be wound about a spool without creating any undesirablestresses on the centrally disposed stress member nor are the signalwires of the signal wire bundle subjected to forces that might tend tobreak them during storage or service conditions. The seismic hydrophonestreamer provided by this invention is of neutral buoyancy and therebyeffectively obviates any necessity for provision of externally connectedfloats or weight devices that might otherwise be necessary to supportthe streamer as it is towed through a body of water at any suitabledepth. The invention also provides a seismic hydrophone streamer that issubstantially incompressible and may be utilized at extreme water depthswithout sacrificing the sensitivity of the hydrophone transducer. Thisis made possible by the extrusion of a syntactic foam about a centralstress member which foam is composed of a thermoplastic material admixedwith air filled microspheres structures. The extrusion processeffectively eliminates high costs of manufacture and results in theprovision of a seismic hydrophone streamer that is capable of readycommercial acceptance because of its low cost. It is readily understood,therefore, that this invention is well adapted to attain all of theobjects and advantages hereinabove set forth together with other objectsand advantages that are inherent in the apparatus itself. While certainrepresentative embodiments and details thereof have been shown for thepurpose of illustrating this invention. it will be apparent to thoseskilled in the art that various changes and modifications may be madetherein without departing from the spirit or scope of this invention.

What is claimed is:

l. A method of manufacturing a neutral buoyancy seismic hydrophonestreamer having a plurality of hydrophone transducers which comprises:

admixing a thermoplastic resin with gas filled microspheres to obtain amixture having a specific gravity less than I;

conveying said mixture to the heating zone of an extruder;

heating said mixture to a temperature sufficient to melt saidthermoplastic resin;

extruding said mixture at a pressure less than about 300 pounds persquare inch about a central streamer assembly comprising a cable-likestress member to produce a syntactic foam extrudate having a centralstress member; and

affixing hydrophone transducers to said extrudate.

2. The method of claim 1 wherein said central streamer assembly alsocomprises signal wires and bydrophone transducers assembled with respectto said central stress member, said mixture is extruded over saidassembly.

3. The method of claim 1 wherein said assembly includes signal wiresaround said member and extending to one end of said streamer includingthe additional steps of forming recesses at intervals on said extrudate;

affixing said transducers to said extrudate in said recesses to producean assembly of hydrophones on said extrudate having a uniform diameter;and interconnecting said transducers and said wires.

4. A method of manufacturing a neutral buoyancy seismic hydrophonestreamer having a plurality of hydrophone transducers which comprises:

admixing a thermoplastic polymer of a bi-functional isocyanate and apolyether glycol, said polymer being extrusible at temperatures of about300 pounds per square inch or less, with gas filled microspheres toobtain a mixture having a specific gravity less than i;

conveying said mixture to the heating zone of an extruder',

heating said mixture to a temperature sufficient to melt saidthermoplastic polymer;

extruding said mixture at a pressure less than about 300 pounds persquare inch about a central streamer assembly comprising a cable-likestress member to produce a syntactic foam extrudate having a centralstress member; and

affixing hydrophone transducers to said extrudate 5. The method of claim4 wherein said thermoplastic polymer is a linear polymer of methylenebis-4- phenyldiisocyanate and polytetramethylene ether glycol.

1. A METHOD OF MANUFACTURING A NEUTRAL BUOYANCY SEISMIC HYDROPHOPHONESTREAMER HAVING A PLURALITY OF HYDROPHONE TRANSDUCERS WHICH COMPRISES:ADMIXING A THERMOPLASTIC RESIGN WITH GAS FILLED MICROSPHERES TO OBTAIN AMIXTURE HAVING A SPECIFIC GRAVITY LESS THAN 1, CONVEYING SAID MIXTURE TOTHE HEATING ZONE OF AN EXTRUDER, HEATING SAID MIXTURE TO A TEMPERATURESUFFICIENT TO MELT SAID THEMOPLASTIC RESIN, EXTTUDING SAID MIXTURE AT APRESSURE LESS THAN ABOUT 300 POUNDS PER SQUARE INCH ABOUT A CENTRALSTREAMER ASSEMBLY COMPRISING A CABLE-LIKE STRESS MEMBER TO PRODUCE ASYNTACTIC FOAM EXTRUDATE HAVING A CENTRAL STRESS MEMBER, AND AFFIXINGHYDROPHONE TRANSDUCERS TO SAID EXTRUDATE.
 2. The method of claim 1wherein said central streamer assembly also comprises signal wires andhydrophone transducers assembled with respect to said central stressmember, said mixture is extruded over said assembly.
 3. The method ofclaim 1 wherein said assembly includes signal wires around said memberand extending to one end of said streamer including the additional stepsof forming recesses at intervals on said extrudate; affixing saidtransducers to said extrudate in said recesses to produce an assembly ofhydrophones on said extruDate having a uniform diameter; andinterconnecting said transducers and said wires.
 4. A method ofmanufacturing a neutral buoyancy seismic hydrophone streamer having aplurality of hydrophone transducers which comprises: admixing athermoplastic polymer of a bi-functional isocyanate and a polyetherglycol, said polymer being extrusible at temperatures of about 300pounds per square inch or less, with gas filled microspheres to obtain amixture having a specific gravity less than 1; conveying said mixture tothe heating zone of an extruder; heating said mixture to a temperaturesufficient to melt said thermoplastic polymer; extruding said mixture ata pressure less than about 300 pounds per square inch about a centralstreamer assembly comprising a cable-like stress member to produce asyntactic foam extrudate having a central stress member; and affixinghydrophone transducers to said extrudate.
 5. The method of claim 4wherein said thermoplastic polymer is a linear polymer of methylenebis-4-phenyldiisocyanate and polytetramethylene ether glycol.